Speckle patterns are interference patterns emitted from target surfaces illuminated by coherent light. If the target surface moves, the associated speckle pattern is moved as well. This physical phenomenon provides the basis for speckle based navigation sensors. Typically, speckle based navigation sensors include a laser light source, optical components and a photodetector. The speckle pattern consists of speckle “beams” that are emitted nearly isotropically from the illuminated target surface
The speckle pattern emitted from an illuminated target surface is made up of quasi-collimated beams of light. The cross sectional diameter of an individual speckle beam is inversely proportional to the diameter of the illuminated spot on the target surface and is proportional to the distance from the target surface. Typically, two photodetector arrays are used in speckle based navigation sensors with each photodetector array sensing motion along one of the axes, respectively. The cross sectional speckle beam size must be matched to the size of the individual photodetector array elements. Hence, the photodetector arrays or collection optics are often positioned at relatively large distances from the target surface and therefore subtend small angles from the target surface. Because speckle “beams” propagate away from the target surface in nearly an isotropic pattern, only a relatively small fraction reaches the photodetector array. This limits the performance of speckle based navigation sensors.
FIG. 1 shows a conceptual view of a conventional speckle based navigation sensor system for two dimensional navigation. Photodetector arrays 120 and 125 detect a fraction of speckle beams 165 from target surface 135. For example, if photodetector arrays 120 and 125 are located approximately 20 mm from target surface 135 illuminated by the laser light, the angles subtended by photodetector arrays 120 and 125 from target surface 135 are about 7×24 degrees. Because speckle is emitted nearly isotropically only about 1% of the speckle flux strikes photodetector arrays 120 and 125, assuming a photodector array area of 2.8×8.6 mm.